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Why physicists still use Fortran (2015)

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Re: Why physicists still use Fortran (2015)

#191

Post by quadruplebond »

gnufx wrote: post #130
quadruplebond wrote: post #11

As someone working on an Exascale project for electronic structure calculations, I have a theory about the longevity of Fortran. It's the fact that many of these codes were started years ago and the people who have the credentials and ability to get funding for super computing projects learned on Fortran and stayed with Fortan because they were scientist first and programmers second. Modern Fortran has many nice feat…

I makes no sense for people to have moved to C for things that have recently appeared in Fortran. What are the features they've been missing which have appeared in Fortran in the last 10 years? So why is NWChem being re-written in C++, and what relationship does that have to exascale? Richard O'Keefe said 10 years ago ago "Why not start by rewriting the Fortran code _in_ Fortran? Fortran 90 is a very pleasant languag…

I don’t use Fortran but I agree as of today and probably post 90 Fortran is nice enough but people moved to c++ for things like polymorphism and templates and pointers among other things. You could probably also rewrite nwchemex in Modern Fortran just fine. I’ll give you one example of why C++ might be easier though. Next gen linear algebra libraries are being written in c/c++ such as elemental and dplasma also Cyclopes tensor framework and probably others. When the libraries you want to use are in c/c++ the that can be the easiest path forward.

Re: Why physicists still use Fortran (2015)

#192

Post by quadruplebond »

jabl wrote: post #91
sseagull wrote: post #16

Earlier quoted context omitted.

Yes, the electronic structure community is rapidly moving away from Fortran. In my opinion, Fortran will fall behind as newer hardware, libraries, etc, will drop Fortran support. Also, newer grad students are all much more interested in C/C++/Python. I think this is in part because the newer languages are widely used outside of science and therefore there is much more documentation and tutorials/guides. Not to mentio…

> Yes, the electronic structure community is rapidly moving away from Fortran. Really? Seems to me that with very few exceptions (e.g. GPAW which is python/C and nwchemEx which I've never heard about until the parent poster mentioned it), electronic structure is pretty much a Fortran bastion. (Source: I did a Phd doing mostly electronic structure calculations, graduated ~5 years ago)

There is a lot of electronic structure in c and c++. For SMP there is psi4 and Orca also Garnet Chan has a new python C++ package. Then some widely used integral code generators generate c or c++ (libcints, Libint2)

On the parallel front MPQC has been around a long time and is C++.

Re: Why physicists still use Fortran (2015)

#194

Post by S4M »

aswanson wrote: post #119
jacquesm wrote: post #108

Earlier quoted context omitted.

That's the whole trick. Good code makes complex stuff look simple. Bad code makes simple stuff look complex.

That quote should be the metric by which code/languages are judged.

How do you measure the complexity of the code, and most importantly, of the problem that the code is solving?

Re: Why physicists still use Fortran (2015)

#195

Post by quadruplebond »

sseagull wrote: post #142
gnufx wrote: post #133

Earlier quoted context omitted.

I don't understand why the implementation language of low level libraries should determine a high level language that uses them. In what way are C libraries inaccessible from Fortran, given that it defines interoperability with C? I'm afraid you need a 10-, or preferably 20-, year perspective, not a week.

I have been developing in Fortran for years. I just mentioned that as an aside. About C compatiblity: Many C libraries use pointers in their interfaces. Interoperability is indeed defined by the Fortran 2003 standard, which I have used several times to wrap existing C libraries. However, much of the existing code is F90 only (some even F77...), and a vast majority of Fortran developers in the field are not familiar w…

It’s partly the power of library authoring that is moving things I think. To my knowlegdge most post lapack tensor algebra is all C/C++ also. I don’t know Fortran but I am not sure if it has the same flexibility when it comes to generic code and things like writing things like template expression math codes.

Finally the fact that groups like Facebook and google are writing their machine learning code in C++ shows that 1 they find it useful and 2 plenty performant. This kinda became a response to the comment above yours sorry.

Re: Why physicists still use Fortran (2015)

#196

Post by quadruplebond »

noobermin wrote: post #129
nejenendn wrote: post #25

Earlier quoted context omitted.

Fortran is easier to optimize compared to c/++ if you don’t use restrict for the c end. If you do use restrict (iirc) the compilers are competitive.

What I don't get is why someone doesn't make a superb template library with C++ that hides the restricts; it would make writing performant codes in C++ easier and you'd get the host of what C++ has to offer

Eigen isn’t good enough? You can offload some stuff to MKL with it also.

Re: Why physicists still use Fortran (2015)

#197

Post by kowdermeister »

scottlegrand2 wrote: post #74
sidlls wrote: post #26

Earlier quoted context omitted.

Yep. Most of my programming classes were either in pascal or c when I was in undergrad, but in my physics courses, all the way through my graduate education, I used FORTRAN. It's because physics doesn't change that much over the years, FORTRAN code that is time-tested and that works exists and there's no need to re-invent the wheel in another language when more interesting and important problems exist to solve.

David Baker's Rosetta code, which made him a pile of money, is IMO a spectacular example of craptastic C++ written by people who really didn't understand C++ but didn't let that deter them from using every single feature of the language, badly. Some years back we tried to port it to CUDA but there were so many levels of indirection, dereferencing, and virtual functions that it was nearly impossible to make any progre…

> which made him a pile of money

May I ask how? I don't see ads or any other obvious monetization channels.

Re: Why physicists still use Fortran (2015)

#198

Post by quadruplebond »

pletnes wrote: post #56
quadruplebond wrote: post #15

Earlier quoted context omitted.

Blas is an interface, All performant blas libraries need to written to specifically take advantage of modern hardware. Things like NetLib exists as reference implementations, they are not high performance. The point I am trying to make is that Numpy is unlikely to still be using the exact same code that was written in the 1970's unless its performance isn't critical. It is making the same function calls but the actua…

Numpy can use optimised BLAS but I believe it ships with an f2c transpiled reference BLAS so it can run (although slower) without Fortran where necessary.

Sure but that will likely be much much slower.

Re: Why physicists still use Fortran (2015)

#199

Post by quadruplebond »

pletnes wrote: post #55
quadruplebond wrote: post #14

Earlier quoted context omitted.

For science codes, where Fortran is still used, the most expensive pieces (think DGEMM Kernel) are largely written in architecture dependent ASM anyways so programming language doesn't have that much of an effect on the most time critical pieces of large HPC programs. If a specific function is important enough it will be hand optimized in a way that the language doesn't really matter. Sometimes that means calling MKL…

DGEMM doesn’t necessarily win over MATMUL for small matrices. Useful in e.g rotation matrices in finite element codes, and lots of other areas. Since matmul can be done with inlined loops, you also avoid the function call overhead, but it looks like a function call which is good for readability.

Yeah small and skinny matrix multiplies are an active area of research.

Re: Why physicists still use Fortran (2015)

#200

Post by quadruplebond »

0xbear wrote: post #143
dagw wrote: post #80

Earlier quoted context omitted.

I don't know. Having "rewrote X kloc of scientific Fortran to modern idiomatic C++" on your CV should get you to the head of the line in many places when looking for a job.

Why would you rewrite something that works well instead of just linking to it?

Because you need to modify or extend it in some nontrivial way.
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